Ann,
Magherally does sound a likely match for Makerally. And there are certainly plenty of Presbyterians in that area. According to the PRONI site, Magherally Presbyterian church has the following records:
Baptisms, 1837-57 and 1860-77 [no baptisms exist from 1877-1900]; marriages, 1845-96; account books relating to stipend, collections and expenses, 1788- 1859; session minutes, 1825-35; lists of committed members, 1818, 1833 and 1839; list of stipend payers, 1820-22.
There is a copy in PRONI (the public record office in Belfast). The rootsireland site (subscription) has the baptisms up to 1877 on their site, but no marriages. However it appears that any relevant marriages were in Scotland anyway.
Thomas appears to have married Jane Pollock on 21.11.1848 in Maybole. (I assume she was a widow and that her maiden name was McClelland). So presumably the second marriage to Mary Barclay was also in Scotland (though I can?t see it).
So I don?t think there are going to be any baptism records for Thomas (or his brother Samuel who I see was staying with him in 1851). Your best bet might be to search the stipend payers list 1820-22 to see if their father was listed. What was the father?s occupation by the way, as that might point to other possible records.
Elwyn